[ncl-talk] test if lat-lon point is within gridded model domain

[Dave Allured - NOAA Affiliate]

Jared,

See whether the function gc_inout will work in your case.  You would need
to get the boundaries of the WRF grid as a spherical polygon.  There may
already be published WRF polygons as shape files; I am not familiar with
that.  Then use the function gc_inout to determine inside or outside for
each obs location.

In essence you would be applying two mask tests to each obs point, because
you also say you want to select within a lat/lon box.  Use gc_inout to test
against the larger WRF grid, and use simple arithmetic comparisons for the
lat/lon box.

Caution, there is a little trap here.  Do not use gc_inout for testing
simple four-sided lat/lon bounding boxes.  You would probably get something
unexpected.  gc_inout uses spherical polygons on the earth's surface.  Such
polygons have curved edges on the simple lat/lon plane.

--Dave


On Mon, Nov 20, 2017 at 4:26 PM, Jared Lee <jaredlee at ucar.edu> wrote:

> Hi, I have a WRF domain on a polar stereographic grid (which also happens
> to span the dateline), and I want to interpolate model data to MADIS
> observation locations that are within my domain. Is there a straightforward
> way to evaluate which observation lat/lon points are outside the WRF grid?
>
> Because it's a polar stereographic grid, doing a first pass to eliminate
> MADIS stations by comparing to the min/max latitude and longitude of the
> WRF grid still leaves a ton of geographic area that would be inside that
> lat/lon box but outside the WRF grid (and that's complicated further by the
> WRF domain spanning the dateline).
>
> I tried using rcm2points to do horizontal interpolation, but that function
> is still interpolating to numerous grid points that are thousands of km
> outside my WRF domain. I also tried feeding wrf_user_ll_to_ij some lat/lon
> values for points well outside my domain, and it gives me nonsense (but
> non-missing) values for nearest grid points.
>
> Anyone have any ideas? Or is there a function that already exists to do
> this that I'm apparently not seeing? It seems like a function to return the
> (i,j) values of the four surrounding grid points would be an ideal way to
> accomplish this and be useful in additional contexts and applications.
>
> Jared
>
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